Potential theory(might be entirely wrong): ichigo's appearance in the one shot is his true shinigami form and his appearance in the thousand year blood war arc was made by the zero division. :
Zangetsu (Zanpakutō spirit) | Bleach Wiki | Fandom
Ichigo's Zanpakuto: Exploring the Mysteries and Powers of Zangetsu in Bleach : r/bleach
Zangetsu (Zanpakutō spirit) | Bleach Wiki | Fandom
bleach ichigo new zangetsu | Bleach manga, Manga, Anime
Bleach 541 Manga Chapter Review "Ichigo's New Zanpakuto " ブリーチ - YouTube